A wheel loader has a traveling system and a hydraulic oil working system. The traveling system is driven by an engine, and the hydraulic oil working system is operated by receiving pressurized oil from a hydraulic pump which is driven by the engine. The engine of a wheel loader is set so that the maximum torque Te of the engine at each engine speed somewhat exceeds the total absorption torque (Th+Tr), which is obtained by adding the full absorption torque Th of the hydraulic working system to the absorption torque Tr of the traveling system at each engine speed in a low-to-medium engine speed range (Te&gt;Th+Tr). A difference .DELTA.T[=Te-(Th+Tr)] between the maximum torque Te of the engine and the total absorption torque (Th+Tr) is set as excess torque .DELTA.T (hereinafter, called excess torque .DELTA.T).
The excess torque .DELTA.T ensures an acceleration to abruptly increase the engine speed from the low-to-medium engine speed range (specifically, a low-to-medium engine horsepower range) to a high engine speed range (specifically, a high engine horsepower range) in response to a hard increase in the pressure on the accelerator pedal provided adjacent a driver seat. The acceleration performance becomes higher as the excess torque .DELTA.T is greater. The traveling system has a speed change lever, with a plurality of speed change positions, located adjacent to the driver seat. The speed of the wheel loader is freely changed among a neutral (stopping) position, a forward position, and a reversing position, and among respective speed positions in the forward and reversing positions by the operator manipulating the speed change lever. In most cases, the hydraulic pump is a fixed displacement type, but a variable displacement type is used in some cases. A more specific explanation follows.
As is shown in FIG. 3, the wheel loader comprises a vehicle body A and a working machine B, which is located at the front portion of the vehicle body A. The working machine B includes a pair of arms B2 and a bucket B4. The base (rear) end of each arm B2 is pivotably attached to the front portion of the vehicle body A by a respective pivot pin, so that the bucket B4 can be freely raised and lowered by a pair of arm hydraulic cylinders B1, each of which is connected between the vehicle body A and an intermediate portion of a respective one of the pair of arms B2. The distal (foremost) ends of the pair of arms B2 are pivotably attached by respective pivot pins to the lower backside of the bucket B4, so that the bucket B4 can be freely pivoted between a backwardly tilted position and a dump position by the bucket hydraulic cylinder B3, one end of which is connected to the vehicle body A and the other end of which is connected through a linkage to the backside of the bucket B4 at a location above the pivot pins for the connection of the arms B2 to the bucket B4.
The vehicle body A also includes a traveling system comprised of: a driver seat A1; an engine (not illustrated); a torque converter (not illustrated); a transmission (not illustrated); a drive shaft (not illustrated); a differential (not illustrated); four tires A2 located at the right front, left front, right rear, and left rear of the vehicle body; a speed change lever (not illustrated); and the like. The vehicle body A further comprises a hydraulic working system which includes: a hydraulic oil circuit, having a hydraulic pump and which makes the hydraulic cylinders B1 and B3 extendable; the working machine B; a working machine operating lever (not illustrated), which is provided adjacent to the driver seat A1; and the like. The engine speed is controlled in response to the depression angle of an accelerator pedal (not illustrated). The traveling system can also include, for example, a steering hydraulic oil circuit, and the like. Further, various kinds of traveling systems can be used, for example, a system with the torque converter replaced by a damper mechanism, a system with the above mechanical traveling system replaced by a full hydraulic type, an electric type, or the like.
The wheel loader, constructed as described above, carries out a sole traveling operation, a sole working operation, and a joint operation which is a combination of the traveling operation and the working operation.
(1) The sole traveling operation is based on the operation of the speed change lever, the accelerator pedal, and the like, without operating the hydraulic working system (specifically, the working machine operating lever is in its neutral position). The sole traveling operation is conducted, for example, while traveling on a pavement, traveling between two sites, traveling a medium distance or a long distance with a load on the bucket B4 (so-called, load-and-carry), or the like.
(2) The sole working operation is based on the operation of the working machine operating lever, the accelerator pedal, or the like, without operating the traveling system (specifically, the speed change lever is in its neutral position, and a brake pedal is operated). The sole working operation is conducted, for example, when raking and excavating natural ground with the bucket B4 while the loader is in a non-traveling (stopped) condition.
(3) The combined operation is based on the simultaneous operations of the working machine operating lever, the speed change lever, the accelerator pedal, etc. For example, the vehicle body A is moved forwardly, the blade edge of the bucket B4 is thrust into natural ground, and then while the working machine lever is being operated, the accelerator pedal is fully depressed. Thus, the purposes of the combined operations include obtaining a resultant force (a vector) of a greater thrusting force and raking force, produced by hydraulic oil pressure, and excavating with strong power at a higher speed, by increasing the discharge quantity of the hydraulic pump.
In the wheel loader conducting the above combined operations, with the speed change operation being by means of the speed change lever, and the oil quantity adjusting operation being by means of the working machine operating lever, the operator depresses the accelerator pedal hard. As for the operation of a wheel loader having an inching pedal or the like, an operation of depressing the inching pedal or the like can be also included. On the other hand, when an excavating force or a traveling force with smaller strength at a lower speed is desired, the operator only slightly depresses the accelerator pedal. Specifically, the operation mainly consists of a simple operation of obtaining engine horsepower corresponding to the load, in accordance with the depression angle of the accelerator pedal.
However, unless the engine horsepower quickly changes in response to an increase in the pressure on the accelerator pedal, excavation and travel with a high efficiency cannot be achieved. Especially in a wheel loader, a depressing operation to abruptly shift the engine speed from a low-to-medium engine speed range to a high engine speed range is frequently carried out by abruptly depressing the accelerator pedal. For example, in the combined operations described supra, when the vehicle body A is moved forwardly and the edge of the blade of the bucket B4 is thrust into natural ground, some vehicle speed is sufficient until the moment at which it is thrust. Specifically, if there is vehicle speed to some extent, it is not necessary to depress the accelerator pedal so hard, that is, it is suitable if the engine speed is in a low-to-medium speed range.
However, when excavation with strong power at a higher speed is to be conducted next by abruptly and fully depressing the accelerator pedal while manipulating the working machine lever, the acceleration performance of the engine is decreased if there is no excess torque .DELTA.T. Specifically, the engine horsepower does not change quickly in response to a change in the pressure on the accelerator pedal, and a combined operation with a high efficiency cannot be achieved. Accordingly, as for the engine of the wheel loader, it is important to secure the excess torque .DELTA.T in a low-to-medium engine speed range.
Japanese Laid-open Patent No. 3-107587 discloses a prior art system of automatically changing the displacement volume of a hydraulic pump in proportion to the engine speed by using a variable displacement type of hydraulic pump, thereby matching the engine torque with the consumption torque (=hydraulic pump absorption torque+torque converter absorption torque) to obtain an energy saving.
However, as the system disclosed in Japanese Laid-open Patent No. 3-107587 automatically changes the displacement volume of the hydraulic pump in proportion to the engine speed, it has nothing to do with acceleration performance. Making it easier to understand, it is an art of automatically changing the displacement volume of the hydraulic pump in proportion to the engine speed when the engine speed is changed, regardless of the acceleration performance. More specifically, according to the "Detailed Description" section and the FIG. 3 in Japanese Laid-open Patent No. 3-107587, the total absorption torque of the fixed pump and the variable pump in the hydraulic working system side and the torque converter of the traveling system side in a low-to-medium engine speed range greatly exceeds the engine torque, and the excess torque even has a negative value. Thus, the system disclosed in the Japanese Laid-open Patent No. 3-107587 has nothing to do with the acceleration performance of the wheel loader.
On the other hand, it is impossible to obtain an acceleration and an energy-saving performance exceeding those in the prior art from a simple operation of obtaining the engine horsepower corresponding to the load, based on the depression angle of the accelerator pedal, which is an ordinary operation of the prior art.